In previous work, we showed that intramuscular (IM) injection of a recombinant adeno-associated viral (AAV) serotype 2 vector expressing F.IX resulted in long-term expression of F.IX in mice and hemophilic dogs. Phase I clinical studies demonstrated that IM injection of AAV-F.IX in humans was safe and resulted in gene transfer and expression as judged by biopsy of injected muscle in all patients tested. Although access to skeletal muscle is easily performed by direct IM injections, achievement of AAV therapeutic target doses in humans has proved impractical because of the large number (> 300 sites) of injections required. Thus there is a great interest in exploiting techniques that allow transduction of large numbers of muscle fibers without requiring hundreds of intramuscular injections. Skeletal muscle contains a rich network of capillaries that can be chemically and/or mechanically modified to ensure vascular leakage of fluid containing vectors. The overall goal of this work is to establish the efficacy and safety of novel regional intravascular delivery methods of vectors to extensive areas of the skeletal muscle of a limb. In aim 1 we will determine the efficacy and safety of two different regional intravascular delivery methods. The first method is isolated limb perfusion (ILP) in which the vector is delivered through the femoral artery. The second approach is anterograde limb perfusion (ALP) in which the vector is injected through a superficial vein in the distal part of the limb under elevated hydrostatic pressure. This goal is encouraged by our preliminary data on these techniques documenting widespread transduction of muscle tissue of the hind limb in normal dogs and by the improvement of the hemophilia B phenotype in a canine model of severe hemophilia B. In aim 2 we will determine whether AAV vectors of alternate serotypes result in higher levels of F.IX expression. These results will have clinical implications in terms of (a) dose-response advantage of non-AAV-2 vectors, and (b) treatment of patients with high titer neutralizing antibodies to AAV-2 (about 20% of general population). In aim 3 we will test whether the use of F.IX variants could improve the efficacy in terms of the amount of F.IX required for the correction of the hemophilia B phenotype. For a disease category in which a substantial percentage (>80%) of severely affected adults have iatrogenic underlying liver disease, a muscle-directed strategy is very attractive. The results of this translational study will establish an experimental basis for clinical studies of this delivery method in humans with hemophilia B and also for gene transfer for other diseases such as muscular dystrophy.